can you run something like iperf3 or openspeedtest between the server and client to prove its a network throughput issue?
do you have a network switch you can add to avoid switching through your router (if it is indeed bad?)
Have you ensured you arent unknowingly using wifi at either end?
I think a lot of people have facebook accounts but dont use it for facebook. Messenger, instagram, marketplace… I’d guess that most people using those services don’t actually “use facebook” and probably think they arent providing profitable data to meta by using those services.